1. FIELD OF THE INVENTION
The present invention relates to a low temperature-sinterable dielectric composition, whose sintered composition, when prepared by firing a uniform mixture consisting of lead ferrotungstate, lead titanate and manganese dioxide at a temperature of not higher than 1,000.degree. C. has the general formula A.Pb(Fe.sub.2/3 W.sub.1/3).sub.1-x Ti.sub.x O.sub.3 +B.MnO.sub.2, wherein 0.005.ltoreq..times..ltoreq.0.65, A=0.95-0.9995 and B=0.0005-0.05, a relative dielectric constant of at least 2,000 at 25.degree. C., a dissipation factor (tan .delta.) of not more than 5% at 25.degree. C., and a specific resistance of at least 10.sup.9 .OMEGA..cm at 25.degree. C., and also to a thick film capacitor having a dielectric layer prepared from said composition.
2. DESCRIPTION OF THE PRIOR ART
Barium titanate so far used as a dielectric substance has a sintering temperature as high as 1,300.degree.-1,400.degree. C., and has the relative dielectric constant of 1,500 at the room temperature and the relative dielectric constant of 6,000 at the curie point (120.degree. C.).
To improve the relative dielectric constant of barium titanate at the room temperature, the following methods (1) and (2) have been used.
(1) Method for reducing grain sizes of barium titanate ceramics to about 1 .mu.m, and
(2) method for lowering the curie point of barium titanate to the room temperature by adding strontium titanate, calcium stannate, barium stannate, barium zirconate, etc. to barium titanate.
However, the dielectric substances obtained according to said methods (1) and (2) have a relative dielectric constant of 2,000-6,000 at 25.degree. C., but still have a high sintering temperature such as 1,300.degree.-1,400.degree. C. Thus, in the production of thick film capacitors (including laminated capacitors), it is necessary to use an expensive metal containing platinum or palladium capable of withstanding the sintering temperature as a main component as electrode.
Thus, a dielectric composition capable of being sintered at a low temperature such as not higher than 1,000.degree. C. will be very preferable owing to the following advantages in the production of thick film capacitors:
(1) A cheaper firing oven than the conventional one will be applicable.
(2) Thermal energy can be saved.
(3) Cheaper metal of silver group can be used as electrode.
As the dielectric composition capable of being sintered at a low temperature such as not higher than 1,000.degree. C., lead-containing complex perovskite type oxide, etc. have been reported (A. I. Agronooskaya: Akad. Nauk. Phys. Serie, 1960, pp 1271-1277). Furthermore, it has been reported that, among said oxides, Pb(Fe.sub.2/3 W.sub.1/3)O.sub.3 (lead ferrotungstate) can be sintered at 880.degree. C., and the resulting sintered product has the relative dielectric constant of 1,800 at 25.degree. C. and the dielectric constant of 8,700 at the curie point (-95.degree. C.) (A. Bokov et al: Soviet Phys. JETP, 1962, Vol. 15, pp 447-449). However, said sintered product has a high dissipation factor at 25.degree. C., such as 16%, and a low specific resistance at 25.degree. C. such as 10.sup.7 .OMEGA..cm, and thus cannot be applied as a dielectric substance.